This is just not true. Jail mail is borderline unethical and many of the lawyers who engage in it are also engaging in disreputable purchasing of inmate lists. Thats not to say all attorneys who do jail mail are unethical as that is a sweeping generalization. It is absolutely NOT the most common way a criminal defense attorney receives clients. Solicitation by attorneys in this way is ethically frowned upon. I suspect that attorneys who advocate it, engage in it and therefore feel the need to defend it. There are plenty of resources like these on the world wide web that provide free information without an overt solicitation that can lead a client to a more well intentioned attorney.
Jail mailers=bottom feeders. If you ever find a good attorney in jail mail you'll stand alone. As Mr. Michaels indicates that is not how most of us get our clients. I don't advertise at all, for example, and get all my clients through referrals, from my website, or sometimes from Avvo answers. I have never approached an inmate I don't know and tried to take him away from his PD or private counsel.
I personally don't do jail mail but know several well respected and established attorneys across California who do and who have for years. Although the jail mail process may not have been on the "up & up" in the past, it was litigated through the court system and certain rules were put in place by the court regarding this process. If it were unethical, the court would have made that ruling. As for such attorneys being "bottom feeders", you're are more than entitled to your opinion. As with any profession, there are good attorneys and bad ones. But making blanket characterizations such as that is just not true.